With the rapid development of display technology, flexible display panel has advantages such as thin type, light weight, shatterproof, flexible, and energy saving; liquid crystal display (LCD) panel has advantages such as low power consumption, high display quality, no electromagnetic radiation, and a wide range of applications. Therefore, flexible liquid crystal display panel has become a hotspot of research in the field of display.
A polyimide (PI) film is typically used in an existing flexible liquid crystal display panel as a flexible substrate, since the PI film is able to withstand high temperature close to 400. In such a manner, when thin film transistors are subsequently manufactured on the PI film, the PI film can not be damaged by the high temperature environment for forming the active layer in the thin film transistor.
When a flexible liquid crystal display panel is in a dark state, an in-plane retardation of a PI film Ro=0 nm; a thickness retardation of the PI film Rth is about 2330 nm. In such a situation, the PI film is equivalent to a negative C plate, of which the retardation in a certain visual angle is
            Γ      ɛ        =                            2          ⁢          π                λ            ⁢              n        o            ⁢              d        ⁡                  (                                                    1                -                                                                            sin                      2                                        ⁢                                          θ                      0                                                                            n                    e                    2                                                                        -                                          1                -                                                                            sin                      2                                        ⁢                                          θ                      0                                                                            n                    o                    2                                                                                )                      ;λ is the wavelength of incident light; no is the refractive index of ordinary light; ne is the refractive index of extraordinary light; d is the thickness of the negative C plate; θ0 is a polar angle (i.e., the angle between the human visual line and the axis perpendicular to the center of the display panel). FIG. 1 is a simulated diagram of a retardation of a negative C plate in different visual angles. As can be seen from FIG. 1, the retardation of the negative C plate changes periodically. The retardation of the negative C plate in different visual angles will cause serious problem of light leakage when a flexible liquid crystal display panel is in a dark state. As can be seen from FIG. 2, regardless of a color filter layer, the transmittance of a flexible liquid crystal display panel in a dark state can be up to 26%. In FIG. 1 and FIG. 2, 0°˜360° represent azimuthal angles; 0°˜90° represent polar angles; the azimuthal angle and the polar angle represent a visual angle together.
Moreover, an angle formula for absorption axes of an upper polarizer and lower polarizer in a flexible liquid crystal display panel is
            cos      ⁢                          ⁢      ψ        =                            -                      sin            2                          ⁢                  θ          k                ⁢                  cos          ⁡                      (                                          ϕ                2                            -                              ϕ                k                                      )                          ⁢                  cos          ⁡                      (                                          ϕ                k                            -                              ϕ                1                                      )                                                            1            -                                          sin                2                            ⁢                              θ                k                            ⁢                                                cos                  2                                ⁡                                  (                                                            ϕ                      k                                        -                                          ϕ                      1                                                        )                                                                    ⁢                              1            -                                          sin                2                            ⁢                              θ                k                            ⁢                                                cos                  2                                ⁡                                  (                                                            ϕ                      2                                        -                                          ϕ                      k                                                        )                                                                          ;ψ is an angle between the absorption axis of the upper polarizer and the absorption axis of the lower polarizer; φ1 and φ2 are respectively azimuthal angles of the upper polarizer and lower polarizer when the visual angle is zero; φk is an azimuthal angle of human eye when the visual angle is greater than zero; θk is a polar angle of human eye when the visual angle is greater than zero. When the visual angle is greater than zero, the absorption axis of the upper polarizer and the absorption axis of the lower polarizer are not perpendicular to each other, which also causes a problem of light leakage when a flexible liquid crystal display panel is in a dark state.